A traveling-wave tube is often used as a transmission source amplifier for a high frequency wave (microwave). The traveling-wave tube is a means for amplifying a high frequency wave (electromagnetic wave) for transmission, through interaction while making it travel in the same direction as an electron beam that is an amplification energy source. With regard to an amplification operation in the traveling-wave tube, it is necessary to divert a high frequency wave of high speed in order to have the speed in direction of travel of the electron beam and of the high frequency wave to be of a similar level. That is, a slow-wave circuit that delays the high frequency wave is necessary.
As a method of delaying a high frequency wave (diverting a high frequency wave), there is a method, for example, in which the high frequency wave is propagated in a helical waveguide, and an electron beam is passed at the center of the waveguide. The helical waveguide portion that diverts the high frequency wave in this way is called a helix slow wave circuit.
Meanwhile, there is presently a strong demand for high frequency waves with regard to wireless frequency. Specifically, research and development of wireless devices in the terahertz range is progressing. With the progress of high frequency waves from microwaves to terahertz waves, since wavelength becomes smaller (since wavelength shortens), miniaturization of “helical wiring” occurs in the abovementioned helix slow wave circuit, and manufacture of the circuit becomes difficult.
Therefore, in the high frequency wave band described above (for example, terahertz range), a “folded waveguide” form, for which microstructure realization is comparatively easy, is viewed as being promising, and research and development is proceeding. In the folded waveguide, a high frequency wave (electromagnetic wave) is made to pass a waveguide bent in meander line form, and is delayed. The traveling-wave tube (waveguide) has a configuration provided with a beam hole so that an electron beam travels (passes through) the center thereof.
Specifically, the folded waveguide has a structure as shown in FIG. 8, with a configuration in which a beam hole 10 passes through the center of the folded waveguide 20. It is to be noted that details of the configuration of the traveling-wave tube provided with the folded waveguide and a stopband described later are disclosed in Non-Patent Literature 1.